The present invention relates generally to methods and systems for measuring a physical quantity using a sensor that receives its power via an optical transmission line and that transmits its measurements via an optical transmission line. More particularly, the present invention relates to a method and system for measuring a physical quantity using at least one sensor that converts the physical quantity into an electrical measuring signal, in which the electrical energy for the sensor is transmitted optically from an electro-optical power converter to an optoelectric power converter and from there to the sensor, in which the measuring signals of the sensor are transmitted in an optical analog manner from an electro-optical signal converter to an optoelectric signal converter and there converted into electrical analysis signals, and in which a working-point current is delivered to the electro-optical signal converter in order to set its working point.
Optical transmission systems for transmitting measuring signals between at least one electrical measurement head with a sensor and a base station having an electronic analysis system are preferably used in cases where either high electrical isolation or high electromagnetic compatibility (EMC)* for the transmission link, or both, are required. Examples of such applications include measuring current in high-voltage facilities and measuring EKG respiration and pulse of a patient in magnetic resonance imaging. FNT *(so called electromagnetic immunity EMI)
Hybrid fiber optic sensor systems in which the measurement head and base station each draw their energy from an electrical power supply unit, and measurement data from the sensor are transmitted via optical fiber from the measurement head to the electronic analysis system, are known. For this purpose, an electro-optical signal converter is provided in the measurement head which converter is supplied with a working-point current for a light-emitting diode (LED) from the power supply unit associated with the measurement head, and which converts the measuring signals into optical signals.
These optical signals are transmitted through a fiber optic bundle or a single optical fiber to an optoelectric signal converter in the base station, and converted back into electrical signals. These electrical signals are generally amplified in an amplifier unit.
If the electrical power supply unit for the measurement head is an external voltage source, electrical lines are required between the voltage source and the measurement head, making sufficient insulation of the measurement head difficult. A battery can therefore be provided in the measurement head as the power supply unit. However, a battery has a limited lifetime and must therefore be replaced regularly. Also known are sensor systems in which electrical energy is obtained from the environment, for example inductively from a power network or with solar energy. However, these energy sources present the problem of insufficient reliability if the current level is zero or the sun is not shining.
In another known hybrid fiber optic sensor system, high insulation and a permanent, reliable energy supply to the measurement head are achieved by the fact that energy for the measurement head, in particular for the sensor and signal converter, is also transmitted optically. For this purpose an optical power source, such as a laser, is provided in the base station, in addition to the electronic analysis system. The optical energy of the optical power source is transmitted via a fiber optic cable to an optoelectric converter in the measurement head, preferably to a specially configured GaAs photoelement array. This converter is then provided in the measurement head as the power supply unit for the sensor and the signal converter. See "Sensors and Actuators A,"25-27 (1991), pp. 475-480.
If analog optical signals are used to transmit the measurement data, the working point of the system (and therefore the measurement data) can be distorted by changes in ambient temperature and by aging phenomena in the electro-optical and optoelectric converters, and by changes in the gain factor and damping in the optical transmission link. For this reason, until now frequency-analog or digital optical signals have been used in the known systems to transmit measurement data. The voltage-frequency converters (VCOs) used to convert analog signals into frequency-analog signals have relatively restricted transmission bandwidths, and require intrinsic energies of approximately 500 mW or more for transmission rates in the MHz range. In addition, the A/D converters which convert analog signals into digital signals are relatively slow, and require considerable intrinsic energy for a desired accuracy of less than 0.1%. By contrast, analog transmission of optical signals has the advantage of a high transmission rate with a large bandwidth as well as a comparatively low energy requirement.
The present invention is directed to the problem of developing a measurement method and a measurement apparatus using analog optical signal transmission in which interference affecting the working point of the transmission system for the measurement data is corrected. In addition, the present invention is directed to the problem of developing such a measurement system and method in which the gain of the measuring signals can be corrected.